1. Field of the Invention
The present invention relates generally to oscillation circuits adaptable to FM modulation circuits employed in home VTR (Video Tape Recorders) and the like.
2. Description of the Background Art
In recording a luminance signal, in a home VTR, the luminance signal is clamped in a clamping circuit to make uniform direct current peak levels of a synchronizing signal thereof, and the clamped luminance signal is then FM modulated in an FM modulation circuit. This FM modulation circuit generally comprises an astable multivibrator. This astable multivibrator oscillates at a carrier frequency in a synchronizing signal period, but has an oscillation frequency changing depending on the level of the luminance signal in the other periods. The described carrier frequency is, for example, set at 3.4 MHz in a VHS system, but is generally required to have high accuracy. Accordingly, such as for implementation of the FM modulation circuit in an IC (an integrated circuit), stabilization of the carrier frequency is attempted by adjusting a variable resistor which is externally provided to the IC. Furthermore, the external variable resistor also adjusts a deviation range of the oscillation frequency. Such an FM modulation circuit is, for example, disclosed in Japanese Utility Model Laying-Open No. 60-150865.
However, since the external variable resistor should be adjusted manually in a method of adjusting the carrier frequency thereby, there are disadvantages of time-consuming adjustment and the increasing number of elements. Moreover, even if the adjustment is carried out once, readjustment is required due to changes in temperature and with the passage of time. In addition, since the adjustments are required for both the carrier frequency and the frequency deviation range, and thus the one adjustment influences the other, it is difficult to carry out both adjustments correctly.